NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | EXAMPLE | COLOPHON
STDARG(3) Linux Programmer's Manual STDARG(3)
stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
#include <stdarg.h>
void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);
A function may be called with a varying number of arguments of varying types.
The include file <stdarg.h> declares a type va_list and defines three macros
for stepping through a list of arguments whose number and types are not known
to the called function.
The called function must declare an object of type va_list which is used by
the macros va_start(), va_arg(), and va_end().
The va_start() macro initializes ap for subsequent use by va_arg() and
va_end(), and must be called first.
The argument last is the name of the last argument before the variable
argument list, that is, the last argument of which the calling function knows
the type.
Because the address of this argument may be used in the va_start() macro, it
should not be declared as a register variable, or as a function or an array
type.
The va_arg() macro expands to an expression that has the type and value of the
next argument in the call. The argument ap is the va_list ap initialized by
va_start(). Each call to va_arg() modifies ap so that the next call returns
the next argument. The argument type is a type name specified so that the
type of a pointer to an object that has the specified type can be obtained
simply by adding a * to type.
The first use of the va_arg() macro after that of the va_start() macro returns
the argument after last. Successive invocations return the values of the
remaining arguments.
If there is no next argument, or if type is not compatible with the type of
the actual next argument (as promoted according to the default argument
promotions), random errors will occur.
If ap is passed to a function that uses va_arg(ap,type) then the value of ap
is undefined after the return of that function.
Each invocation of va_start() must be matched by a corresponding invocation of
va_end() in the same function. After the call va_end(ap) the variable ap is
undefined. Multiple traversals of the list, each bracketed by va_start() and
va_end() are possible. va_end() may be a macro or a function.
An obvious implementation would have a va_list be a pointer to the stack frame
of the variadic function. In such a setup (by far the most common) there
seems nothing against an assignment
va_list aq = ap;
Unfortunately, there are also systems that make it an array of pointers (of
length 1), and there one needs
va_list aq;
*aq = *ap;
Finally, on systems where arguments are passed in registers, it may be
necessary for va_start() to allocate memory, store the arguments there, and
also an indication of which argument is next, so that va_arg() can step
through the list. Now va_end() can free the allocated memory again. To
accommodate this situation, C99 adds a macro va_copy(), so that the above
assignment can be replaced by
va_list aq;
va_copy(aq, ap);
...
va_end(aq);
Each invocation of va_copy() must be matched by a corresponding invocation of
va_end() in the same function. Some systems that do not supply va_copy() have
__va_copy instead, since that was the name used in the draft proposal.
The va_start(), va_arg(), and va_end() macros conform to C89. C99 defines the
va_copy() macro.
These macros are not compatible with the historic macros they replace. A
backward compatible version can be found in the include file <varargs.h>.
The historic setup is:
#include <varargs.h>
void
foo(va_alist)
va_dcl
{
va_list ap;
va_start(ap);
while (...) {
...
x = va_arg(ap, type);
...
}
va_end(ap);
}
On some systems, va_end contains a closing '}' matching a '{' in va_start, so
that both macros must occur in the same function, and in a way that allows
this.
Unlike the varargs macros, the stdarg macros do not permit programmers to code
a function with no fixed arguments. This problem generates work mainly when
converting varargs code to stdarg code, but it also creates difficulties for
variadic functions that wish to pass all of their arguments on to a function
that takes a va_list argument, such as vfprintf(3).
The function foo takes a string of format characters and prints out the
argument associated with each format character based on the type.
#include <stdio.h>
#include <stdarg.h>
void
foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch (*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\n", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
}
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2001-10-14 STDARG(3)